In the field of semiconductor manufacturing, optical lithography has been the mainstream approach used in patterning semiconductor devices. In typical prior art lithography processes, UV light is projected onto a silicon wafer coated with a thin layer of photosensitive resist (photoresist) through a mask that defines a particular circuitry pattern. Exposure to UV light, followed by subsequent baking, induces a photochemical reaction which changes the solubility of the exposed regions of the photoresist. Thereafter, an appropriate developer, usually an aqueous base solution, is used to selectively remove photoresist either in the exposed regions (positive-tone photoresists) or, in the unexposed regions (negative-tone photoresists). The pattern thus defined is then imprinted on the silicon wafer by etching away the regions that are not protected by the photoresist with a dry or wet etch process.
One type of photoresist employed in the prior art is a chemically amplified photoresist which uses acid catalysis. A typical prior art chemically amplified photoresist, for example, is formulated by dissolving an acid sensitive polymer and a photoacid generator in a casting solution. A chemically amplified photoresist is especially useful when relatively short wavelength radiation is employed, including deep UV radiation 150-315 nm wavelengths, and mid-UV radiation, e.g., 350-450 nm wavelengths. The shorter wavelengths are typically desired to increase resolution, and thus, decrease feature size of the semiconductor devices, but fewer photons are radiated for a given energy dose.
Accordingly, higher exposure doses are typically required when using UV radiation to obtain a sufficient photochemical response in the photoresist unless a chemically amplified photoresist is employed. In a chemically amplified photoresist, acid sensitivity of the base polymer exists because acid sensitive side chain groups are bonded to the polymer backbone. When such a photoresist is exposed to radiation, the photoacid generator produces an acid that, when the photoresist is heated, causes catalytic cleavage of the acid sensitive side chain groups. A single acid catalyst molecule generated in this manner may be capable of cleaving multiple side chain groups, thus allowing lower exposure doses for the needed photochemical response.
Because of the relatively low intensity of ArF laser source and relatively high binding energy of acid labile moieties in ArF photoresist, photoacid generators which can produce stronger Bronsted acid with much higher sensitivity are preferred to realize such chemical amplification in commercial lithography. Fluorine-containing onium salts, such as perfluoronated octyl sulfonate (PFOS) and perfluoronated alkyl sulfonate (PFAS), are generally preferred used as the photoacid generator in ArF photoresist system in part because they result in generation of strong acid.
In recent years, there has been a desire in the microelectronics industry to eliminate the use of perfluorinated carbons (PFCs) such as PFOS and PFAS. Thus, there is a desire to find alternative photoacid generators which can be used without adversely impacting the performance of lithographic processes. There has also been a desire to minimize or eliminate fluorine content in photoresist in order to improve etch resistance and to improve process latitude in high numeric aperture (NA>0.95) imaging processes.
Some attempts have been made to develop photoresist formulations that do not use perfluorinated carbon-containing photoacid generators, however these have largely been unsuccessful in achieving performance comparable to formulations using PFOS. Thus, there is a need for new and improved photoacid generators and chemically amplified photoresist compositions that enable the substantial reduction or avoidance of PFCs and/or fluorine in photoresist compositions.